Retinal vessel diameters of 45 diabetic children and adolescents with diabetes were measured on black-and-white fundus photographs (negatives) using digital image analyses technique and microdensitometry as well as with direct micrometry. The changes in vessel diameters in two sets of photographs taken at an interval of 2.4 +/- 0.3 years were compared with glycated haemoglobin level, retinopathy status and various other parameters. During the follow-up period both arterial and venous diameters increased significantly (p = 0.0436 and p = 0.0001, respectively), but the changes in individual vessels and between patients differed markedly. The superior temporal vein dilated more than the inferior temporal vein. The average increase in venous calibre was related to the glycated haemoglobin level on the date of the second examination (p = 0.0492). Signs of early retinopathy developed more often in the eyes with venous dilation (6 of 18 eyes) than in those without (3 of 24 eyes), but the difference did not reach statistical significance. The results indicate that retinal vasodilation is associated with chronically elevated blood glucose level in diabetic children, and may precede development of other signs of retinopathy.